Doppler ultrasound monitoring systems operate on the principal that ultrasound energy incident upon a moving surface is reflected by that surface and experiences a change of frequency which is proportional to the rate of movement of the surface. The most widespread medical applications for doppler ultrasound monitoring systems have occurred in blood flow analysis (the ultrasound is reflected from flowing blood) and heart rate analysis (the ultrasound is reflected from one or more appropriate surfaces of the heart).
Often, it is desirable to utilize multiple ultrasound transducers so that measurements can be taken at two different locations or from two different angles. Typically, this has engendered substantial expense or substantial complications which affect the reliability of measurements. While ultrasound transducers are relatively inexpensive, the electronic equipment which drives the transducer and senses the signal that the transducer produces can be relatively complex and expensive. Furthermore, the electronics associated with different probes can have characteristics which are quite different, such as different degrees of phase shift. Moreover, these differences can change with environmental conditions, such as temperature and with time. Accordingly, the electronics can require frequent or constant calibration in order to assure accurate results.
Basically, it is an object of the present invention to avoid the shortcomings of existing multi-transducer Doppler ultrasound monitoring systems. It is specifically an object to reduce substantially the amount of electronic equipment required to operate a plurality of ultrasonic sensors in a Doppler ultrasound monitoring system.
It is also an object of the present invention to eliminate the need for relative calibration of the electronics utilized to drive a plurality of ultrasound transducers and to sense the signals produced thereby.
It is another object of the present invention to provide a multi-transducer Doppler ultrasound monitoring system which is convenient and reliable in use, yet relatively inexpensive and simple in operation.
In accordance with the present invention, a multiplexed switch is interposed between a plurality of ultrasound transducers and a single transmitter and receiver of the type utilized to drive a single transducer and to process the signal produced by that transducer, respectively. This switch has a plurality of inputs, each of which receives a connection to one of the transducers, and a single output which is connected to the transmitter and receiver, and it is effective to connect each input, in sequence, to the output on a time-shared basis. In addition, a track & hold circuit is provided at the output of the electronics. This circuit has a single input and a plurality of outputs, each corresponding to one of the multiplex switch inputs. The track & hold circuit senses the signal at the output of the electronics and distributes it in sequence to each of the outputs. In this manner, a single set of electronics can be utilized for a plurality of transducers, thereby eliminating the cost of additional electronics, as well as the need for relative calibration of the electronics for different transducers.